The present invention relates to a cryogenic freezing apparatus and method in which articles are cooled within a freezing chamber of a cryogenic freezer by application of a cryogen supplied to the freezing chamber as a liquid. More particularly, the present invention relates to such a cryogenic freezing apparatus and method in which the cryogen vaporizes in the course of the freezing of the articles and is thereafter recovered through recompression, purification and liquefaction.
Cryogenic freezers are well known in the art and have principal application to the food processing industry for refrigerating food by either lowering the temperature of the food or by actually freezing the food. Such freezers are also used within industrial processes in which the temperature of articles are lowered in the course of such processes. In such freezers, the articles to be cooled are brought into direct contact with a cryogen and the cryogen vaporizes to form a cryogenic vapor.
Cryogenic spiral belt freezers are very common in the food processing industry. In spiral belt freezers, an endless spiral belt receives the food from an inlet plenum and discharges the food from an outlet plenum. The inlet and outlet plenums can be located at the same or at different levels in which case the spiral belt freezer is termed as either ascending or descending. Within the cryogenic spiral belt freezer, one or more injection nozzles spray liquid cryogen, such as carbon dioxide which immediately expands into carbon dioxide snow, onto the food. The food cools upon contact with the cryogen and the cryogen boils off into a vapor.
As may be appreciated, the cryogenic vapor tends to flow out of the inlet and outlet plenums. Since cryogenic vapor such as carbon dioxide or nitrogen can pose a danger of suffocation, the cryogenic vapor is conventionally expelled from the work area by a blower and suitable conduits. It is to be noted that the expelling of the cryogen vapor is uneconomical as a waste of refrigerant. In order to prevent the cryogen from being wasted, a recovery unit can be employed in conjunction with the cryogenic freezer to purify and then liquefy the cryogenic vapor drawn from the inlet and outlet plenums. After liquefaction, the cryogen can be recycled for use in freezing articles such as food.
The use of a recovery unit within a cryogenic spiral belt freezer is illustrated in U.S. Pat. No. 3,728,869. In this patent, a freezing chamber is provided with enlarged inlet and outlet plenums termed therein as entrance and exit vestibules. The interior of the freezing chamber is kept at a positive pressure and carbon dioxide vapor is drawn at the inlet and outlet vestibules, also at a positive pressure by a recirculating fan. The inlet and outlet vestibules are sufficiently deep such that the dense carbon dioxide filling the vestibules produces a barrier that prevents the ingress of air into the freezing chamber. The carbon dioxide vapor drawn from the inlet and outlet vestibules can be recirculated back to the freezing chamber after being reliquefied.
Another example of the use of a recovery unit used in conjunction with a cryogenic freezer is shown in U.S. Pat. No. 4,952,223, the specification and drawings of which are hereby incorporated by reference. The recovery unit illustrated in this patent is designed to recompress, purify and liquefy carbon dioxide vapor obtained from a carbon dioxide freezer and to recycle the resultant liquid carbon dioxide back to the freezer. Unlike the examples given above, carbon dioxide vapor is recovered from the top of a freezer along with air present within the freezer. In fact, air can be injected into the air and carbon dioxide flow drawn from the freezer to assist evacuation of the freezer. Moreover, there is no provision for the prevention of air entry into the freezer. Even so, this patent makes it very clear that the lower the concentration of carbon dioxide present in the feed to be recovered, the lower the recovery rate of the carbon dioxide.
In fact with any recovery unit, it is important that the concentration of air within the air/cryogen mixture being fed to the recovery unit be as low as possible to increase the recovery rate of the cryogen. It goes without saying that the more cryogen lost from the freezing unit, the less cryogen there will be to recover. There are, however, practical problems associated with optimizing the recovery rate of a cryogen after its vaporization within a cryogenic freezer. For instance, conventional spiral belt freezers can not easily be adapted to utilize the disclosure contained within the '869 patent without major redesign. The reason for this is that inlet and outlet plenums of conventional spiral belt cryogenic freezers would not collect a sufficient amount of cryogenic vapor to form an air barrier. Cryogen recovery from the inlet and outlet plenums of conventional cryogenic freezers suffers the same infirmities as the '223 patent because air is drawn along with the cryogenic vapor to the recovery unit.
As will be discussed, the present invention provides a cryogenic freezer employing a cryogenic vapor recovery unit which processes the cryogenic vapor essentially free of air while minimizing loss of the cryogen from the freezer in order to enhance the recovery rate of the cryogen.